This will be a multicenter, prospective, randomized, comparative study of daily and twice-weekly Trimethoprim/Sulfamethoxazole (TMS) in HIV-infected patients with either a prior history of PCP or a CD4+ cell count of < 200 mm3 or < 15 percent of the total lymphocyte count. Patients will be randomized evenly to one of the two prophylaxis regimens.